Method and Device for Optimizing the Utilization of an Engine

ABSTRACT

A method of optimizing the use of an aircraft power plant having at least one engine operating within a performance envelope covering at least a first rating and a second rating, the first rating presenting a first power (P1) usable over a predetermined first time interval (D1), the second rating presenting a second power (P2) greater than said first power (P1), the second power (P2) being usable continuously over a predetermined second time interval (D2). Thus, while the engine is developing a third power (P3) that is both greater than the first power (P1) and less than or equal to the second power (P2), a potential first duration of utilization (ΔT) of continuous use of the second power (P2) is determined and displayed, the first duration of utilization (ΔT) elapsing at a speed that is variable and that depends on said third power (P3).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/204,938, filed Aug. 8, 2011, now U.S. Pat. No. ______; which claimsthe benefit of FR 10 03478, filed Aug. 31, 2010; the disclosures ofwhich are incorporated in their entirety by reference herein.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a method and a device for optimizingthe utilization of an engine, in particular an engine of an aircraft ofthe rotorcraft type.

(2) Description of Related Art

Most currently-built rotorcraft are fitted with one or two free turbineengines. Power is then taken from a low pressure turbine referred to asa “free” turbine, which low pressure turbine is mechanically independentof the assembly of the engine that comprises the compressor and the highpressure stage, and in particular a high pressure turbine. The freeturbine of a turbine engine generally operates at a speed of rotationlying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, soa speed-reduction gearbox is needed in the connection with the mainrotor of the rotorcraft since its speed of rotation lies substantiallyin the range 200 rpm to 400 rpm: this is the main gearbox.

Thermal limitations of a turbine engine, and torque limitations of amain gearbox serve to define a performance envelope covering two normalutilization ratings for a turbine engine arranged on a single ortwin-engined rotorcraft:

takeoff rating corresponding to a torque level for the gearbox andheating for the turbine engine that can be accepted for a limited lengthof time without significant degradation: this is the maximum takeoffpower, written PMD by the person skilled in the art, that is usable fora continuous duration of five minutes for example and over anaccumulated duration of thirty minutes in a single mission; and

the maximum continuous rating during which, at no time, are thecapabilities of the gearbox or the capabilities that result from themaximum acceptable continuous heating of the high pressure blades of thefirst stage of the turbine exceeded: this is the maximum continuouspower, referred to as PMC by the person skilled in the art, and it canbe used without any time limit, corresponding to about 90% of the PMD.

On a twin-engined rotorcraft, the performance envelope also coversemergency contingency ratings that are used only when only one of thetwo turbine engines has failed:

a first emergency rating during which the capabilities of the gearboxconcerning its inlet stages and the thermal capabilities of the turbineengine are used to the maximum: this is referred to as super contingencyrating and it is equal to about 112% to 120% of PMD, being usable for acontinuous duration of thirty consecutive seconds at the most, forexample, with this being possible three times during a flight, beingreferred to as PSU where OEI stands for “one engine inoperative”. If PSUis used, then it is necessary to remove and overhaul the turbine engine;

a second emergency rating during which the capabilities of the gearboxconcerning its inlet stages and the capabilities of the turbine engineare used very largely: this is referred to as maximum emergency powerand is equal to about 105% to 110% of PMD, and is referred to as PMU orOEI2′ by the person skilled in the art since it is usable for twoconsecutive minutes, for example; and

a third emergency rating during which the capabilities of the gearboxconcerning its inlet stages and the thermal capabilities of the turbineengine are used without damaging them: this is referred to asintermediate emergency power and is equal to PMD, being referred to bythe person skilled in the art as continuous OEI or PIU or OEI2, since itcan be used continuously for the remainder of the flight after failureof a turbine engine.

Consequently, the thermal and mechanical constraints and above all thephenomenon of turbine blade creep give rise to degradation of theturbine engine to a greater or lesser extent depending on the rating. Inorder to guarantee safe flight and obtain good performance, it istherefore essential to determine the maximum acceptable level of damagefor a turbine engine.

Consequently, an overall utilization potential is evaluated for theturbine engine. Specifically, this amounts to defining a maximum numberof flying hours known as time between overhauls (TBO), that the turbineengine is capable of performing since its most recent overhaul or itsfirst use, depending on circumstances. Once TBO has been reached, theturbine engine is removed and then overhauled.

In the text below, and for convenience, the term “most recent overhaulof the turbine engine” is used to designate either the first use of theturbine engine or in fact the most recent overhaul thereof.

Furthermore, in order to obtain flying authorization in a given countryfor a rotorcraft, it is required that the performance envelope and theTBO of the turbine engine(s) of the rotorcraft to be certified by theofficial services in the country in question for a precise spectrum ofutilizations. Such authorization is thus given only after completecertification tests have been completed, which are expensive.

Since these complete certification tests of a turbine engine areperformed in order to justify a performance envelope associated with aTBO, it is not possible to use the turbine engine in some otherperformance envelope that differs from the initially authorizedperformance envelope without again performing complete certificationtests, which are very expensive.

In addition, when the engine develops an intermediate power between themaximum continuous power PMC and the maximum takeoff power PMD, thisintermediate power is subjected to the same limitation as the maximumtakeoff power.

Likewise, when the engine is developing power lying between theintermediate emergency power and the maximum emergency power, that poweris processed in the same manner as the maximum emergency power, beingsubjected to the same limitation as said maximum emergency power.

Under such circumstances, operation of the turbine engine would appearnot to be optimized because of the discrete staging of engine ratings.

The state of the technical prior art includes document FR 2 878 288 inwhich it is proposed to modify the maximum number of flying hours inorder to optimize use of the engine.

Furthermore, document FR 2 888 287 makes it possible to define aperformance envelope that is an alternative to the initial performanceenvelope of the engine.

SUMMARY OF THE INVENTION

An object of the present invention is thus to propose a method and adevice that enables the utilization of an engine to be optimized.

The present invention thus relates to a method of optimizing the use ofan aircraft power plant having at least one engine operating within aperformance envelope covering at least a first rating and a secondrating, the first rating presenting a first power usable over apredetermined first time interval, and the second rating presenting asecond power greater than the first power, the second power being usablecontinuously over a predetermined second time interval.

This method is remarkable in particular in that while the engine isdeveloping a third power that is both greater than the first power andless than or equal to the second power, a potential first duration ofutilization of continuous use of the second power is determined anddisplayed, e.g. in the form of a countdown, the first duration ofutilization elapsing at a speed that is variable and that depends onsaid third power.

For an aircraft applying the method, the method does not imply changingthe maximum number of flying hours TBO for which the engine iscertified.

Furthermore, with an engine for an aircraft, e.g. a rotorcraft, themethod is compatible with the certification rules in force insofar as itdisplays the first utilization duration that is potentially availablefor the second certified power. The flying manual of the aircraft isthus unchanged.

In the state of the art, when the engine is developing a third power, itis assumed that the engine is operating at the second rating that isusable for the second time interval.

In the invention, the pilot is informed of a first utilization durationthat is available for the second power at said second rating, with thisfirst utilization duration elapsing at a speed that is variable anddifferent from the speed at which time elapses as defined by theinternational system (SI) for measuring time. This first utilizationduration is thus a first counter that counts down so that the pilot canestimate the remaining duration of utilization at the second power.

Thus, one second of the first utilization duration is longer than orequal to the second defining the unit of time in said internationalsystem for measuring time.

When the third power is less than the second power, utilization of thethird power does less damage to the engine than would utilization of thesecond power. The second time interval can therefore be increased, withthis increase being represented by a first utilization duration for thesecond power that elapses more slowly than real time.

Thus, the invention makes it possible to use a third power that is lessthan the second power for a length of time that is longer than thesecond time interval. This method amounts to creating a new ratingdefined by the third power and by a third time interval, but without anyneed to modify the flying manual.

As a result, with the first power being the maximum continuous power PMCand the second power being the maximum takeoff power PMD, it is possiblefor example to hover or climb fast using a rotorcraft while developingengine power that lies between the first power PMC and the second powerPMD for a length of time that is longer than the second time interval.

Similarly, on a twin-engined rotorcraft, it is possible to use anemergency rating lying between the second and third emergency ratingsfor a length of time that is optimized in order to become extricatedfrom an awkward situation.

Furthermore, the method may include one or more of the followingadditional characteristics.

For example, in order to determine the first utilization duration:

during a preparatory stage, it is possible to establish a deteriorationcurve of an engine, the curve providing a coefficient of deteriorationof the engine as a function of the value of a monitoring parameter ofthe engine, which parameter may be the temperature of the gas at theentry to the free turbine in a free turbine engine, and to establish atotal damage level caused by a use of the second power during the secondtime interval, the total damage level being equal to the product of thesecond time interval multiplied by a targeted deterioration coefficientdetermined, with the help of the curve, by using the value of themonitoring parameter reached while the engine is developing the secondpower; and

in real time, in flight, establishing and storing in memory the currentdeterioration coefficient of the engine, the first duration ofutilization ΔT at each current instant being obtained using thefollowing first relationship:

${\Delta \; T} = \frac{{{EP}\; 2} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{K\; 2}$

where:

K2 represents said targeted deterioration coefficient;

EP2 represents said total damage; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding topassing from the first power towards the third power and said currentinstant TPS.

It can be understood that the first utilization duration isreinitialized at the end of utilization of the third power on startingthe power plant, and also on each occasion power goes from the firstpower to the third power (non-accumulated time measurement that isreinitialized on each excursion).

Furthermore, the closer the third power comes to the second power, thefaster the first duration of utilization elapses.

When the third power is less than the second power, one second of thefirst duration of utilization is longer than the unit of timemeasurement of the international system.

In contrast, when the third power is equal to the second power, thefirst duration of utilization elapses at the same speed as real time,with one second of the first utilization duration being equal to thetime measurement unit of the international system.

Furthermore, the power plant may have two engines, with the firstduration of utilization determined for each engine, and the smallerfirst duration of utilization may be displayed from among the firstutilization duration of the first engine and the first utilizationduration of the second engine.

In another aspect, a warning may be triggered when said first durationof utilization becomes less than a first predetermined threshold inorder to attract the attention of an operator of a vehicle provided withthe invention, e.g. an audible or visible alarm.

In another aspect, a second potential duration of continuous utilizationof a third power developed at a current instant may be determined anddisplayed e.g. in the form of a countdown, the third power being bothgreater than the first power and also less than or equal to the secondpower, said second duration of utilization elapsing at a rate that isvariable, depending on said third power.

Thus, it is possible to know how much time remains during which it ispossible to use this third power without impacting the lifetime of theengine.

In order to determine the second utilization duration the followingsteps are performed:

during a preparatory stage, establishing a deterioration curve of anengine, the curve providing a coefficient of deterioration of the engineas a function of the value of a monitoring parameter of the engine, andestablishing a total damage level caused by a use of the second powerduring the second time interval, the total damage level being equal tothe product of the second time interval multiplied by a targeteddeterioration coefficient determined, with the help of the deteriorationcurve, by using the value of the monitoring parameter reached while theengine is developing the second power; and

in real time, in flight, establishing and storing in memory the currentdeterioration coefficient of the engine, the second duration ofutilization at each current instant being obtained using the followingfirst relationship:

${\Delta \; T} = \frac{{{EP}\; 2} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{K\; ({TPS})}$

where:

K(TPS) represents said deterioration coefficient at the current instant;

EP2 represents said total damage; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding topassing from the first power towards the third power and the currentinstant TPS.

It can be understood that the second utilization duration isreinitialized at the end of using the third power on starting the powerplant.

This second utilization duration is then a second counter that runs soas to allow the pilot to estimate the duration of utilization thatremains for the third power.

In addition, for a power plant having two engines with the secondutilization duration being determined for each engine, it is possible todisplay the utilization duration among the second utilization durationfor the first engine and the second utilization duration for the secondengine.

According to another aspect, it is possible to trigger a warning whenthe second utilization duration becomes less than a second giventhreshold in order to attract the attention of an operator of a vehiclefitted with the invention, e.g. an audible or visual alarm.

In another aspect, said second power can be used for an accumulated timeTC2, longer than the second time interval, but in which each period ofcontinuous utilization cannot exceed said second time interval, a thirdpotential utilization duration, in accumulated and discontinuous time,of said second power is determined and displayed, e.g. in the form of acountdown, while the engine is developing during current utilization athird power that is both greater than the first power and also less thanor equal to the second power, said third utilization duration beingdetermined:

during a preparatory stage, by establishing a deterioration curve of anengine to provide a deterioration coefficient of the engine as afunction of the value of a monitoring parameter of the engine, and byestablishing an overall damage level caused by utilization of the secondpower during an accumulated time TC2, said overall damage being equal tothe product of the accumulated time TC2 multiplied by a targeteddeterioration coefficient determined, with the help of said curve, byusing the value of the monitoring parameter reached while the engine isdeveloping the second power;

at the end of each continuous utilization of the third power for anintermediate duration, by storing an intermediate damage level E_(i) inmemory, said level being determined using the following integral of thedeterioration coefficient K(t) as a function of time during saidintermediate duration:

${Ei} = {\int\limits_{0}^{di}{{{K(t)} \cdot \delta}\; t}}$

in real time during the flight, establishing and storing in memory thecurrent deterioration coefficient of the engine, the third duration ofutilization ΔT″ at each current instant being obtained by using thefollowing third relationship:

${\Delta \; T^{''}} = \frac{{{EPC}\; 2} - \left\lbrack {{\sum\limits_{i = 1}^{n - 1}{Ei}} + {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}} \right\rbrack}{K\; 2}$

where:

K2 represents said targeted deterioration coefficient;

EPC2 represents said overall damage level;

$\sum\limits_{i = 1}^{n - 1}{Ei}$

represents the stored accumulated intermediate damage acquired duringutilizations of the third power preceding the current utilization; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding topassing from the first power to the third power during the currentutilization and said current instant TPS.

It can be understood that the intermediate level of damage and the thirdutilization duration ΔT″ are reinitialized on starting the power plant.

In addition to a method, the invention also provides piloting meansapplying the method.

According to the invention, piloting means for an aircraft power planthaving at least one engine operating within a performance envelopecovering at least a first rating and a second rating, said first ratingpresenting a first power usable over a predetermined first timeinterval, said second rating presenting a second power greater than saidfirst power, the second power being usable continuously over apredetermined second time interval, the second power possibly also beingusable over an accumulated time interval that is longer than the secondtime interval but for which each period of utilization must be shorterthan the second time interval, which means are remarkable in particularin that they comprise:

determination means for determining a first possible continuousutilization duration of the second power when the engine is developing athird power that is both greater than the first power and less than orequal to the second power, the determination means determining the firstutilization duration:

-   -   with the help of a deterioration curve of an engine, the curve        providing a coefficient of deterioration of the engine as a        function of the value of a monitoring parameter of the engine,        and establishing a total damage level caused by a use of the        second power during the second time interval, the total damage        level being equal to the product of the second time interval        multiplied by a targeted deterioration coefficient determined,        with the help of the curve, by using the value of the monitoring        parameter reached while the engine is developing the second        power; and    -   in real time, in flight, establishing and storing in memory the        current deterioration coefficient of the engine, the first        duration of utilization at each current instant being obtained        by the determination means using the following first        relationship:

${\Delta \; T} = \frac{{{EP}\; 2} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{K\; 2}$

where:

-   -   K2 represents the targeted deterioration coefficient;    -   EP2 represents the total damage; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding topassing from the first power towards the third power and the currentinstant TPS; and

display means co-operating with the determination means to display theremainder of the first utilization duration.

The piloting means may be incorporated in a first limitation instrumentof an aeroengine, in particular.

BRIEF DESCRIPTION OF THE DRAWING

The invention and its advantages appear in greater detail in the contextof the following description of implementations given by way ofillustration with reference to the accompanying figures, in which:

FIG. 1 is a diagram explaining the piloting means of the invention;

FIG. 2 is a diagram explaining the method of the invention;

FIG. 3 is a diagram showing a deterioration curve; and

FIG. 4 is a diagram showing the deterioration coefficient as a functionof time.

Elements present in more than one of the figures are given the samereferences in each of them.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows piloting means 2 for optimizing the use of engines 1, 1′ ina power plant of an aircraft, for example.

Each engine is dimensioned and certified to operate in a plurality ofstaged operating ratings. For example, each engine may operate in afirst rating in which the engine may develop a first power P1 during afirst time interval D1, e.g. an infinite time interval, and in a secondrating during which the engine may develop a second power P2 during alimited second time interval D2 on a continuous basis.

For example, each engine is a turbine engine, with the first ratingbeing the maximum continuous power rating during which the enginedevelops a maximum continuous power PMC, with the second rating being atakeoff rating during which the engine develops a maximum takeoff powerPMD for a maximum of 5 minutes continuously, for example.

With a power plant such as that shown diagrammatically that has twoengines, the first rating may be the emergency third rating in which theengine develops an intermediate emergency power for a one engineinoperative (OEI) power level without time limit, the second rating maybe the second emergency rating during which the engine develops amaximum emergency power that is usable for two consecutive minutes, forexample (OEI2′).

Under such circumstances, the first power P1 may be the maximumcontinuous power PMC, in which case the second power P2 is the maximumtakeoff power PMD, or indeed the first power P1 may be an intermediateemergency power OEI, in which case the second power P2 is the maximumemergency power OEI2′.

The piloting means 2 may include determination means 3 for determining afirst possible duration of utilization ΔT that is continuously usablefor the second power P2, while the engine is developing a third power P3greater than the first power P1 and less than or equal to the secondpower P2. The determination means 3 may optionally be provided with acalculation member 3′ making use of a memory 3″ for establishing thefirst duration of utilization in application of precise criteria.

Furthermore, the piloting means 2 possess display means 4 for presentingon a screen 5 the first duration of utilization. The display means 4optionally include warning means 6 for triggering a warning when thefirst duration of utilization is less than a first predeterminedthreshold.

For a power plant having only one engine, the display means then displayin particular the first duration of utilization ΔT, this first durationof utilization ΔT running at a speed that varies as a function of thethird power P3 being developed by the engine 1.

For a power plant having a plurality of engines, the determination means3 determine the first duration of utilization ΔT associated with eachengine, the display means then displaying either the first duration ofutilization ΔT of each engine, or else the smallest first duration ofutilization ΔT.

It should be observed that the piloting means 2 shown include singledetermination means 3 common to both engines. Nevertheless, it should beunderstood that the piloting means 2 could include respectivedetermination means for each engine, for example.

FIG. 2 explains the optimization method of the invention.

During a preparatory stage 10, in a first step 11, a deterioration curveCO of an engine is established, which deterioration curve CO provides adeterioration coefficient K of the engine as a function of the value ofa monitoring parameter of said engine.

Any other similar means, e.g. a data table, suitable for giving saiddeterioration coefficient as a function of the value of a monitoringparameter of the engine can be envisaged.

By way of example, the curve may be stored in the memory 3″ of thedetermination means 3.

In addition, when the engine optionally includes a high pressure turbinelocated upstream from a free turbine, the monitoring parameter may bethe temperature of the gas at the inlet to the high pressure turbine,known to the person skilled in the art as the turbine entry temperature(TET).

The blades of the high pressure turbine of the turbine engine aresubjected to centrifugal force and to the temperature TET. Above acertain threshold, the material constituting the blades is the subjectof “creep” and the blades are then subject to deformation, namely to anincrease in the length of the blades. Thus, the blades run the risk oftouching the casing of the high pressure turbine and of thus beingdegraded. The temperature TET is thus associated directly withdegradation of the turbine engine.

Nevertheless, since the temperature TET is very difficult to measurebecause of its relatively non-uniform nature, the monitoring parameteris preferably the temperature of the gas at the entry to the freeturbine, known to the person skilled in the art as T4. Since thistemperature is a good representation of the temperature TET, it isrepresentative of the degradation of the turbine engine.

The monitoring parameter may be the torque developed by the engine orindeed the speed of rotation of a rotary gas generator of the engine, orit may be a function of a plurality of parameters such as the gastemperature at the entry to the free turbine modulated by the outsidetemperature and the outside pressure, for example.

FIG. 3 is a graph plotting a curve CO that determines a deteriorationcoefficient K. The value of a monitoring parameter of the turbine engineis plotted along the abscissa axis and the value of the deteriorationcoefficient K is plotted up the ordinate axis. It should be observedthat the curve CO may be obtained by testing on each model of engine.

Preferably, when the turbine engine has a free turbine, the monitoringparameter is the temperature T4 of the gas at the entry to the freeturbine. This temperature T4 is a good representation of the state ofthe turbine engine since damage thereto is caused mainly by excessivelyhigh temperatures. The greater the temperature of the turbine engine,the more it is degraded. This observation also lies behind theexponential shape of the curve CO.

The graph shows the value of the monitoring parameter T4PMD and theassociated deterioration coefficient KPMD relating to takeoff rating.Under such circumstances, the second power P2 gives rise to adeterioration coefficient K2 that is equal to the associateddeterioration coefficient KPMD when said second power P2 is the takeoffpower TOP.

Similarly, the graph shows the value of the monitoring parameter T4OEI2′and the associated deterioration coefficient KOEI2′ relating to thesecond emergency rating. Under such circumstances, the power P2 inducesa deterioration coefficient K2 equal to said associated deteriorationcoefficient KOEI2′ when said second power P2 is the maximum emergencypower OEI2′.

With reference to FIG. 2, during a second step 12, the deteriorationcoefficient K2 is determined for at least one second possible rating ofthe operating envelope of the engine, e.g. the above-mentioneddeterioration coefficient KPMD or KOEI2′.

With reference to FIG. 2, during a third step 13, for at least onesecond possible rating of the operating envelope of the engine, a totaldamage level EP2 caused by use of the second power P2 of the secondrating during the second time interval D2 is established, with thistotal damage EP2 being equal to the product of the second time intervalD2 multiplied by a targeted deterioration coefficient K2 as determinedwith the help of said deterioration curve CO, by using the value reachedby said monitoring parameter when the engine is developing said secondpower P2, i.e.:

EP2=D2*K2

For example, total damage ETOP associated with takeoff rating isestablished, this total damage ETOP being given by:

EPMD=DPMD*KPMD

where:

EPMD represents the total damage EP2 generated by using the maximumtakeoff power during a second time interval D2, e.g. 5 minutes;

DPMD represents said second time interval D2 during which the takeoffmaximum power is used continuously; and

KPMD represents the targeted deterioration coefficient obtained usingthe deterioration curve CO.

Similarly, total damage EOEI2′ associated with the second emergencyrating is determined, this total damage EOEI2′ being given by;

EOEI2′=DOEI2′*KOEI2′

where:

EOEI2′ represents the total damage EP2 generated by using the emergencymaximum power during a second time interval D2, e.g. 2 minutes;

DOEI2′ represents said second time interval D2 during which the maximumemergency power is used continuously; and

KOEI2′ represents the targeted deterioration coefficient obtained usingthe deterioration curve CO.

The deterioration coefficient K2 of each rating and the total damage EP2of each rating may be stored in the memory 3″ of the determination means3, for example.

In real time, and during a stage of flight 20, when the engine developsa third power P3 greater than the first power P1 of a first rating andless than or equal to the second power P2 of a second rating, thepiloting means 2 calculate and display the first duration of utilizationΔT.

Thus, the determination means 3 receive the current value of themonitoring parameter of the engine and establish the currentdeterioration coefficient K(t) of the engine using said current value ofthe monitoring parameter and the deterioration curve CO. Thedetermination means store the current deterioration coefficient K(t) asfrom the first instant T0 corresponding to going from the first power P1towards the third power P3.

FIG. 4 shows an example of how the current deterioration coefficientK(t) of an engine might vary between this first instant T0 and thecurrent instant TPS.

With reference to FIG. 2, during a first calculation step 21, thedetermination means 3 calculate in real time the time integral of thecurrent deterioration coefficient K(t) between the first instant T0 andthe current instant TPS.

After that, during a second calculation step 22, the determination meansdetermine the first duration of utilization ΔT at each current instantTPS using the following first relationship:

${\Delta \; T} = \frac{{{EP}\; 2} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{K\; 2}$

where:

K2 represents said targeted deterioration coefficient of the secondrating in question;

EP2 represents said total damage of the second rating in question; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding topassing from the first power P1 towards the third power P3 and saidcurrent instant TPS.

When the second rating is takeoff rating, then:

${\Delta \; T} = \frac{{EPMD} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{KPMD}$

Similarly, when the second rating is the second emergency rating, then:

${\Delta \; T} = \frac{{{EPOEI}\; 2^{\prime}} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{{KOEI}\; 2^{\prime}}$

Thereafter, during a display step 24, the determination means 3 causesthe first duration of utilization ΔT to be displayed in the form of acountdown at a rate that varies as a function of the third power P3.

The closer the third power P3 comes to the second power P2, the greaterthe speed at which the first duration of utilization ΔT elapses, withone second of the first duration of utilization ΔT nevertheless beinglonger than the time measurement unit of the international system. Itshould be recalled that since the thirteenth general conference onweights and measures, the second that represents said time measurementunit is no longer defined relative to a year, but relative to a propertyof matter: with this unit being defined since 1967 in the internationalsystem in the following terms: “The duration of 9,192,631,770 periods ofthe radiation corresponding to the transition between the two hyperfinelevels F=3 and F=4 of the 6S1/2 ground state of the cesium-133 atom”.

In order to ensure that the counter giving the second duration ofutilization ΔT indicates zero on starting from two minutes, it may benecessary for three minutes to elapse, for example.

In contrast, when the third power P3 is equal to the second power P2,the first duration of utilization ΔT elapses at the same speed as realtime, with one second of the first duration of utilization ΔT beingequal to the time measurement unit of the international system.

In parallel, during a step 23, the piloting means 2 determine anddisplay in the form of a countdown a second duration of utilization ΔT′that may be performed continuously at a third power P3 being developedat a current instant TPS, the third power P3 being firstly greater thanthe first power P1 and secondly less than or equal to the second powerP2, the second duration of utilization ΔT′ depending on the third powerP3.

In order to determine said second duration of utilization ΔT′, in realtime and in flight, the determination means 3 determine the secondduration of utilization ΔT′ at each current instant TPS using thefollowing second relationship:

${\Delta \; T^{\prime}} = \frac{{{EP}\; 2} - {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}}{K({TPS})}$

where:

K(TPS) represents said deterioration coefficient at the current instant;

EP2 represents said total damage; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding topassing from the first power P1 towards the third power P3 and saidcurrent instant TPS.

Thereafter, during a display step 25, the determination means 3 requirethe second duration of utilization ΔT′ to be displayed in the form of acountdown that varies as a function of the third power P3.

When the power plant has two engines 1, 1′, the second duration ofutilization ΔT′ is determined for each of the engines 1, 1′, and thesmaller of the second durations of utilization ΔT′ is displayed.

Finally, the piloting means 2 may trigger a warning when the secondduration of utilization ΔT′ becomes less than a second predeterminedthreshold.

In addition, during a step 26, the piloting means 2 determine anddisplay in the form of a countdown a third duration of utilization ΔT″of the second power that is possible in accumulated and discontinuousmanner while the engine is developing a third power that is both greaterthan the first power and less than or equal to the second power.

During a preparatory stage, using the deterioration curve CO, overalldamage EPC2 is established due to use of the second power during anaccumulated time interval TC2, this overall damage being equal to theproduct of the accumulated second time interval multiplied by thetargeted deterioration coefficient as determined with the help of thedeterioration curve by using the value of the monitoring parameterreached while the engine is developing the second power.

Furthermore, excursions at third powers lying between the first powerand the second power are discrete and they are numbered using an indexi. The intermediate damage generated during each excursion of anintermediate duration di is equal to:

${Ei} = {\int\limits_{\; 0}^{di}{{{K(t)} \cdot \delta}\; t}}$

Each intermediate damage Ei is established and stored during the flightby the determination means 3.

Under such circumstances, at each current use of the third power, takingplace after earlier excursions that have caused intermediate damage tobe stored in memory, the determination means 3 establish and store thecurrent deterioration coefficient of the engine, and then determine athird duration of utilization ΔT″ at each current instant using thefollowing relationship:

${\Delta \; T^{''}} = \frac{{{EPC}\; 2} - \left\lbrack {{\sum\limits_{i = 1}^{n - 1}{Ei}} + {\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}}} \right\rbrack}{K\; 2}$

where:

K2 represents said targeted deterioration coefficient;

EPC2 represents said overall damage under the effect of using the secondpower during an accumulated duration TC2;

$\sum\limits_{i = 1}^{n - 1}{Ei}$

represents the accumulated intermediate damage stored prior to thecurrent utilization, i.e. during the (n−1) excursions at powers greaterthan the first power and less than or equal to the second power; and

$\int\limits_{T\; 0}^{TPS}{{{K(t)} \cdot \delta}\; t}$

represents the integral of the current deterioration coefficient K(t) asa function of time taken between a first instant T0 corresponding to thebeginning of the current utilization, i.e. the n^(th) passage from thefirst power to the third power and said current instant TPS.

Naturally, the present invention may be subjected to numerous variationsas to its implementation. Although several implementations aredescribed, it will readily be understood that it is not conceivable toidentify exhaustively all possible implementations. It is naturallypossible to envisage replacing any of the means described by equivalentmeans without going beyond the ambit of the present invention.

1. A method of optimizing the use of an aircraft power plant having atleast one engine, the method comprising: operating the engine within aperformance envelope covering at least a first rating and a secondrating, the first rating presenting a first power (P1) which the enginecan develop over a predetermined first time interval (D1), the secondrating presenting a second power (P2) greater than the first power (P1)which the engine can develop continuously over a predetermined secondtime interval (D2) shorter in duration than the first time interval;operating the engine to develop a third power (P3) greater than thefirst power (P1) and less than the second power (P2); while the engineis developing the third power (P3), determining a potential firstduration of utilization (ΔT) of the engine to develop continuously thesecond power (P2) which takes into account engine deterioration causedwhile the engine is developing the third power (P3) relative to enginedeterioration which would be caused if the engine developed the secondpower (P2) instead of the third power (P3) whereby the first duration ofutilization (ΔT) is a third time interval longer in duration than thesecond time interval and shorter in duration than the first timeinterval, and displaying on a display the first duration of utilization(ΔT) in the form of a countdown which elapses at a speed that isvariable and that depends on the third power (P3) relative to the secondpower (P2) such that a pilot can be apprised from the countdown of aremaining duration that the engine can be operated to developcontinuously the second power (P2); controlling the operation of theengine using information provided by the countdown; and wherein themethod amounts to creating for the performance envelope a third ratingpresenting the third power (P3) which the engine can developcontinuously over the third time interval with the first rating, thesecond rating, and time between overhauls (TBO) for which the engine iscertified remaining unchanged.
 2. (canceled)
 3. The method according toclaim 1, wherein the closer the third power (P3) comes to the secondpower (P2), the faster the first duration of utilization (ΔT) elapses.4. (canceled)
 5. The method according to claim 1, wherein the powerplant has two engines and the first duration of utilization (ΔT) isdetermined for each engine, with the smaller first duration ofutilization (ΔT) being displayed.
 13. An aircraft comprising: a powerplant having at least one engine, the engine operating within aperformance envelope covering at least a first rating and a secondrating, the first rating presenting a first power (P1) which the enginecan develop over a predetermined first time interval (D1), the secondrating presenting a second power (P2) greater than the first power (P1)which the engine can develop continuously over a predetermined secondtime interval (D2) shorter in duration than the first time interval, theengine operating to develop a third power (P3) greater than the firstpower (P1) and less than the second power (P2); a processor fordetermining, while the engine is developing the third power (P3), apotential first duration of utilization (ΔT) of the engine to developcontinuously the second power (P2) which takes into account enginedeterioration caused while the engine is developing the third power (P3)relative to engine deterioration which would be caused if the enginedeveloped the second power (P2) instead of the third power (P3) wherebythe first duration of utilization (ΔT) is a third time interval longerin duration than the second time interval and shorter in duration thanthe first time interval; a display for displaying the first duration ofutilization (ΔT) in the form of a countdown which elapses at a speedthat is variable and that depends on the third power (P3) relative tothe second power (P2) such that a pilot can be apprised from thecountdown of a remaining duration that the engine can be operated todevelop continuously the second power (P2); and wherein the processordetermining the potential first duration of utilization (ΔT) amounts tocreating for the performance envelope a third rating presenting thethird power (P3) which the engine can develop continuously over thethird time interval with the first rating, the second rating, and timebetween overhauls (TBO) for which the engine is associated remainingunchanged.
 14. The aircraft according to claim 13, wherein the closerthe third power (P3) comes to the second power (P2), the faster thefirst duration of utilization (ΔT) elapses.
 15. The aircraft accordingto claim 13, wherein the power plant has two engines and the firstduration of utilization (ΔT) is determined for each engine, with thesmaller first duration of utilization (ΔT) being displayed.
 16. A devicefor an aircraft power plant having at least one engine operating withina performance envelope covering at least a first rating and a secondrating, the first rating presenting a first power (P1) which the enginecan develop over a predetermined first time interval (D1), the secondrating presenting a second power (P2) greater than the first power (P1)which the engine can develop continuously over a predetermined secondtime interval (D2) shorter in duration than the first time interval, thedevice comprising: a processor for determining, while the engine isdeveloping a third power (P3) greater than the first power (P1) and lessthan the second power (P2) a potential first duration of utilization(ΔT) of the engine to develop continuously the second power (P2) whichtakes into account engine deterioration caused while the engine isdeveloping the third power (P3) relative to engine deterioration whichwould be caused if the engine developed the second power (P2) instead ofthe third power (P3) whereby the first duration of utilization (ΔT) is athird time interval longer in duration than the second time interval andshorter in duration than the first time interval; a display fordisplaying to a pilot the first utilization duration (ΔT) in the form ofa countdown which elapses at a variable speed dependent on the thirdpower (P3) relative to the second power (P2) such that when controllingthe operation of the engine the pilot can be apprised from the countdownof a remaining duration that the engine can be operated to developcontinuously the second power (P2); a controller for controlling theoperation of the engine using information provided by the countdown; andwherein the processor determining the potential first duration ofutilization (ΔT) amounts to creating for the performance envelope athird rating presenting the third power (P3) which the engine candevelop continuously over the third time interval with the first rating,the second rating, and time between overhauls (TBO) for which the engineis associated remaining unchanged.
 17. The device according to claim 16,wherein the closer the third power (P3) comes to the second power (P2),the faster the first duration of utilization (ΔT) elapses.
 18. Thedevice according to claim 16, wherein the power plant has two enginesand the first duration of utilization (ΔT) is determined for eachengine, with the smaller first duration of utilization (ΔT) beingdisplayed.